Such a process has numerous applications in widely varying fields such as the food industry, e.g. for the fractionation of triglycerides obtained from butter or fish oil, the separation of added high value biomolecules such as vitamins, dyes and pigments, and the polymers industry for the fractionation of polymers of interest to peak industries as silicones, etc.
For all these applications, the starting products are solid or liquid phases having a complex composition requiring the use of numerous fractionation stages in order to bring about the isolation of molecules interest.
More specifically, the invention relates to the joint use of supercritical fluid extraction methods and nanofiltration methods in order to permit the extraction of molecules of interest from complex products, which has hitherto never been possible.
Thus, supercritical fluid extraction methods have hitherto been used up to now for extracting compounds of interest from solid or liquid products such as plants, e.g. for obtaining perfume extracts from various plants or for recovering the active principles, as is described in numerous documents such as the work of E. Stahl "Extraction dense gases for extraction and refining", 1987. Following the extraction operation, the extract is used as it is or it is separated by distillation processes.
Membrane separation methods have been used in various fields such as gaseous diffusion isotope separation, sea water desalination, protein separation, etc., but they have never been combined with a supercritical fluid extraction unit.
However, research has been carried out on the filtration of supercritical fluids such as CO.sub.2, e.g. for separating supercritical CO.sub.2 from ethanol on an asymmetrical kapton polyimide membrane (Semenova et al, 1992), and for separating CO.sub.2 from polyethylene glycol 400 on SiO.sub.2 or polyimide membranes (Nakamura et al, "Membrane separation of supercritical fluid mixture", pp 820-822 in Developments in Food Engineering, published by T. Yamo, R. Matsuno and K. Nakamura Blacku Academic & Professional (Chapman & Hall), London, New York, Tokyo, 1994).
Thus, no research has been carried out on the use of nanofiltration for selectively separating solutes contained in a supercritical fluid.
In nanofiltration methods, use is made of a nanofiltration membrane, which retains the substances having a molecular weight higher than the membrane cutoff threshold, which is in the molecular weight range from 50 to 1000 Daltons, but which allows the passage of substances having a molecular weight below said threshold, in order to separate substances as a function of their molecular weight. This property has never been used with supercritical fluids, because in all the studies carried out the products to be separated are retained by the membrane, even if they have a low molecular weight, such as is the case with ethanol.